The invention relates to an improved process for the preparation of copolymers by polymerization of a mixture of monomers containing about 85 to 25 parts by weight of styrene and/or .alpha.-methylstyrene, about 15 to 75 parts by weight of acrylonitrile and/or methacrylonitrile, and possibly minor quantities of one or more other monomers, which polymerization is carried out in an aqueous dispersion in the presence of one or more compounds yielding free radicals.
This process is well known and has been applied in large scale proportion. However, it yields copolymer products frequently containing considerable quantities of non-converted monomers, up to a few percent depending on the polymerization conditions. The monomer residues are highly undesirable inasmuch as they have an adverse affect on the properties such as gloss and heat resistance, of the end product copolymer. Furthermore, these non-converted monomers may be released from the copolymer product during its subsequent processing or in its end use application, which is highly undesirable in view of their toxicity and the undesirable environmental impact.
Because of these shortcomings of this known process, intensive research has been carried out over a number of years to find methods by which the monomer content of such polymers could be reduced. This research has resulted in a number of proposed solutions, but none are entirely satisfactorily.
For instance, it has been proposed to treat the polymer dispersion with steam for the purpose of stripping part of the non-converted monomers from the polymer. This treatment may be effective, particularly with a protracted treatment, but it is expensive with regard to both investment cost and energy consumption.
Another proposal has been to carry out the removal of monomers in a so-called degasifying extruder. The polymer is melted in an extruder with the simultaneous build-up of high pressure. The product is then introduced into a zone where the pressure is greatly reduced with the result that the volatile monomer components are vaporized and exhausted. However this process requires a very high initial investment and the capacity of such a degasifying extruder is necessarily low if a high removal efficiency is to be achieved.
U.S. Pat. No. 3,991,136 proposes another solution wherein, after the polymerization has proceeded to a degree of conversion of about 90%, a new monomer is added which is very reactive with the non-converted monomer in the final polymer. However, this new monomer should be added in a quantity of five to ten percent by weight with the result that it may substantially influence or affect the ultimate properties of the polymer produced. Furthermore, the addition of such a new monomer would involve relatively high costs resulting from the additional investments and operations it requires.
It is an object of the present invention to provide an improved process yielding polymers of a very low monomer content without the necessity of adding additional monomers.
It is a further object of this invention to provide a solution to the problem of removing non-converted monomer from polymers without adversely influencing or affecting the properties of the final polymer product.
It is yet another object of this invention to reduce the environmental impact and possible risks which might result from monomer emission during production, processing and/or use of a polymer having a relatively high monomer content.
And it is also an object of the invention to increase the degree of conversion of monomer to polymer, thereby increasing the efficiency of polymerization of such monomers by increasing the capacity of the polymerization reactors and utilizing monomers in a more efficient way.